There is already known a photoelectric converting apparatus such as a radiation image pickup apparatus which reads a charge produced by photoelectric conversion in a photoelectric converting element of a sensor array utilizing amorphous silicon or polysilicon, by transfer of the charge to a capacitance by means of a matrix drive.
FIG. 9 is a schematic circuit diagram showing a prior photoelectric converting apparatus (radiation image pickup apparatus). In the prior photoelectric converting apparatus, as shown in FIG. 9, a pixel indicated by a broken-lined frame includes a PIN photodiode PD and a selecting thin film transistor (TFT) ST, and a number of such pixels are arranged two-dimensionally to constitute a sensor array 101. Such device is constituted for example of an amorphous silicon layer and a polysilicon layer, formed on a glass substrate 102. The PIN diodes PD of the pixels receive, at common electrodes thereof, a bias voltage Vs from a power source.
Also gate electrodes of the selecting TFTs ST of the pixels are connected to common gate lines Vg1-VgM. The common gate lines Vg1-VgM are connected to a gate driver 104 provided with a shift register (not shown). Source electrodes of the selecting TFTs ST of the pixels are connected to common signal lines Sig1-SigN. The common signal lines Sig1-SigN are connected to a readout circuit 103 provided with amplifiers Amp1-AmpN, an analog multiplexer MUX and an A/D converter (not shown).
The prior photoelectric converting apparatus of such structure executes a matrix drive by the gate driver 104, whereby photographed image data are outputted to the readout circuit 103 and are read out.
In the following, there will be explained a cross-sectional structure of a sensor array employed in the prior photoelectric converting apparatus such as a radiation image pickup apparatus. FIG. 10 is a cross-sectional view showing a pixel of a prior photoelectric converting apparatus (X-ray image pickup apparatus).
On a glass substrate 201, in each pixel, a gate electrode layer (lower electrode) 202, an insulation layer (amorphous silicon nitride film) 203, an amorphous silicon semiconductor layer 204, an amorphous n-silicon layer 205, and a source/drain electrode layer (upper electrode) 206 are laminated to constitute a selecting thin film transistor (TFT) 222. Also on the glass substrate, an extended portion (lower electrode) of the source/drain electrode layer 206, an amorphous p-silicon layer 207, an amorphous silicon semiconductor layer 208, an amorphous n-silicon layer 209 and an upper electrode layer 210 are laminated to constitute a photodiode 221. Further on the glass substrate 201, there is present a wiring portion 223 constituted by laminating the insulation layer 203, the amorphous silicon semiconductor layer 204, the amorphous n-silicon layer 205, and the source/drain electrode layer 206. Also a protective layer 211 constituted for example of an amorphous silicon nitride film is so formed as to cover these components, and a phosphor layer (not shown) is adhered thereon by an adhesive layer 212. Such structure is described for example in Japanese Patent Application Laid-open No. H08-116044.
The phosphor layer is provided for converting incident radiation (X-rays) into visible light. In general, a photodiode formed with amorphous silicon has an extremely low sensitivity to X-rays. The phosphor layer is constituted for example of a gadolinium-based material or of CsI (cesium iodide).
In such prior photoelectric converting apparatus (X-ray image pickup apparatus), X-rays transmitted by an object are converted, upon entering the phosphor layer, into visible light. Then the visible light enters the photodiode. The photodiode generates a charge in the semiconductor layer, and such charges are transferred, when the TFTs are turned on, to the readout circuit in succession and are read out.
However, in the prior photoelectric converting apparatus such as the radiation image pickup apparatus, a large parasitic capacitance is generated in the common signal line as the number of pixels arranged two-dimensionally increases, thus leading to a significant decrease in the output voltage. More specifically, as shown in FIG. 9, a parasitic capacitance Cgs is present between the gate and source electrodes of the selecting TFT ST, and the magnitude of the parasitic capacitance associated with a common signal line increases in proportion to the number of pixels connected to such common signal line. For example, in the case of preparing an area sensor corresponding to an X-ray film with a size of 40×40 cm by arranging pixels of 200×200 μm each by 2000 units in the longitudinal direction and by 2000 units in the lateral direction, even a parasitic capacitance Cgs of 0.05 pF in one location leads to a parasitic capacitance of 0.05×2000=100 pF per common signal line.
On the other hand, the photodiode PD has a sensor capacitance C of about 1 pF. Therefore, for a signal voltage V1 generated in the photodiode in response to the entry of a visible light, an output voltage V0 observable on the common signal line becomes Vo=V1×Cs/(Cs+Cgs)×2000, whereby the output Vo becomes about 1/100 of the signal voltage V1.
Therefore, the prior photoelectric converting apparatus such as the radiation image pickup apparatus cannot be constructed into a sensor of a large area because of this significant loss in the output voltage. Also because of this loss in the output voltage, such apparatus is susceptible to the influence of noise generated in the amplifiers of the readout circuit and of external noise, whereby a photoelectric converting apparatus of a high sensitivity may be difficult to construct. The influence of such noise may be reduced by providing the readout circuit with a constant current power source or low-noise amplifiers, but such low-noise amplifier, being a special circuit, leads to drawbacks such as an increased cost. Also, since such low-noise amplifier generally has a high electric power consumption, the readout circuit will cause non-negligible heat generation.